In recent years, various natural polysaccharides, such as naturally occurring starch, cellulose, chitin/chitosan, and derivatives thereof, have attracted attention as biomass materials, relative to conventional petroleum-based resins, with increasing concerns about environmental issues. In addition, base materials formed by biodegradable resins, which are decomposable into water and carbon dioxide in the environment, also attract attention and are commercially available. Specific examples thereof include aliphatic polyesters yielded by microorganisms, various polysaccharides such as naturally-occurring starch, cellulose, chitin/chitosan, and derivatives thereof, biodegradable resins completely obtained by chemical synthesis, and polylactic acids obtained by polymerizing lactic acid derived from a starch or other raw materials.
Among the above, the cellulose, which is yielded in the largest quantity on earth, attracts attention as a functional material, because the cellulose is fibriform, has a high crystallinity, a high strength, and a low linear expansion coefficient, and is excellent in chemical stability and living body safety. Particularly, a fine cellulose fiber has been applied in a paper-strengthening agent, a filter aid, a food additive, or the like, and actively developed in recent years.
As a manufacturing method of the fine cellulose fiber, Patent Document 1, for example, discloses a fibrillating (pulverizing) method in which a cellulose suspension is sprayed under a high-pressure of 100 MPa or higher and thereby reducing the pressure.
Patent Document 2 discloses a method for obtaining a fine cellulose fiber by dispersing a cellulose in a medium, the cellulose having carboxyl groups obtained by partially oxidizing hydroxyl groups of the cellulose using a TEMPO (2,2,6,6-tetramethylpiperidinoxy radical) catalyst. The method makes it possible to obtain, with relative ease, the fine cellulose fiber having a type I cellulose crystalline structure by utilizing electronic repulsion of the carboxy groups having negative electrical charges.
Patent Document 3 disclose a method for obtaining an epoxy resin composite by adding a modified fine cellulose fiber to an epoxy resin, the modified fine cellulose fiber being obtained by treating a fine cellulose fiber with an organic onium compound, the fine cellulose fiber being obtained by dispersing a cellulose in a medium, the cellulose having carboxyl groups obtained by partially oxidizing hydroxyl groups of the cellulose using a TEMPO catalyst.
Patent Document 4 discloses a method for obtaining a gas barrier composite compact by coating and drying a gas barrier material on a PET film or a base material made of polylactic acid or the like, the gas barrier material containing a fine cellulose fiber having an average fiber diameter of 200 nm or less being prepared by dispersing in water an oxidized cellulose obtained by a TEMPO oxidation treatment.